Cerebellar tonsillar ectopia may result from altered cerebrospinal fluid (CSF) dynamics and can mimic Chiari I malformation. Distinguishing pressure-related tonsillar descent from a congenital malformation is crucial. We report a case demonstrating dynamic changes in the level of tonsillar ectopia corresponding to variations in CSF dynamics.

A 5-year-old boy with idiopathic intracranial hypertension (IIH) on acetazolamide and history of mastoidectomy presented with worsening headache. Brain MRI revealed 13 mm descent of the cerebellar tonsils below the foramen magnum and diffuse pachymeningeal enhancement. Further evaluation identified transtemporal CSF leakage through the prior mastoidectomy site into the soft tissues of the face and neck. Surgical repair is planned.

 Review of prior imaging showed a 5 mm tonsillar descent during his initial IIH presentation at age 3, accompanied by sixth nerve palsy and a right mastoid lesion, which on biopsy revealed sclerotic trabeculae. Lumbar puncture demonstrated an elevated CSF opening pressure of 36 cm H₂O, confirming IIH. Follow-up MRIs after treatment and during a subsequent episode of bacterial meningitis showed resolution of tonsillar ectopia.

This case illustrates that cerebellar tonsillar ectopia can dynamically shift secondary to both intracranial hypertension and hypotension. Although cerebellar tonsillar herniation due to CSF leakage has been previously reported, the initial Chiari-like tonsillar descent associated with IIH remains controversial. The neuroimaging evidence of IIH and the concurrent improvement in symptoms and imaging findings following treatment suggest that the descent was a manifestation of IIH. We propose that this dynamic shifting of tonsillar position resulted from varying degrees of CSF leakage, with altered CSF flow through the soft tissues producing changes in pressure gradients and flow direction.

Fluctuating CSF dynamics can cause transient tonsillar ectopia in children with intracranial pressure disorders. Serial neuroimaging and close clinical monitoring are essential for accurate diagnosis and management.